June 11, 2011

Tevatron Particle Claims Rejected

Data that hinted at the possible discovery of a new sub-atomic particle has been rejected after cross-checks failed to find support for the initial observation, according to a recent BBC News report.

Researchers working on the CDF experiment at the US Tevatron "atom smasher" announced in May that they had detected exciting hints of a possible unanticipated particle.

But in a separate experiment --called DZero -- cross-checks of the data lacked any good evidence of the findings. The DZero result came days after a CDF team member presented an update showing the signal had strengthened -- not disappeared -- after analyzing nearly twice the amount of data.

Scientists working on both Tevatron experiments are now left with trying to reach a consensus on the issue.

The cross-checks by DZero were revealed in a seminar at Fermilab on Friday. A paper describing the findings is set to be published on the Arxiv repository, and has been submitted to the Physical Review Letters journal for further review.

"We looked at the data-set that CDF originally published... We inject a signal in our simulation which looks like what we would have observed if CDF had seen the real thing," Professor Stefan Soldner-Rembold, spokesperson for the DZero collaboration, told BBC News.

"We analyzed the data accordingly and, as observed, there is no enhancement. We can exclude something like what CDF observed to a relatively high probability," said Soldner-Rembold.

Dr Giovanni Punzi, co-spokesperson for the CDF collaboration at Fermilab, told BBC News: "It has taken a step forward and a step back. But [DZero] has used only half of the data we currently have. So they are now showing a result with our old sample size."

The CDF team analyzed data from collisions between protons and their anti-matter counterparts antiprotons. In these collisions, particles known as W bosons are produced, along with a pair of "jets" of other particles.

It was in these jets that the unexpected "bump" in the data came to light, potentially representing a particle that the theory of particle physics -- known as the Standard Model -- does not anticipate.

And confirmation of the results would have signaled a radical change in physics. But researchers stress the finding is definitely not the elusive Higgs boson -- otherwise known as the "God particle" -- which explains why other particles have mass. The Higgs is the last missing particle needed to complete the Standard Model.

When CDF first broke the news of their potential finding, it had a "three sigma" level of certainty -- meaning there was a 1 in 1000 chance that the result is attributable to some statistical fluctuation in the data.

Punzi announced at a conference in Blois, France on May 30 that the findings were more at a five sigma level of certainty -- meaning there is about a one-in-a-million chance that the "bump" is just a fluke and is the level generally required for a formal discovery.

According to some theorists, the CDF data peak could have provided support for a fifth fundamental force of nature known as "Technicolor" -- which is similar to "strong force," which binds particles known as quarks together inside the nuclei of atoms. It could also give particles their mass - making the Higgs boson unnecessary.

Other researchers suggested an effect called "top background" could explain away the "bump" seen by CDF -- suggesting that researchers might have underestimated the number of top quarks - a fundamental heavy particle - being produced at the Tevatron and that this could have yielded the data peak.

Punzi, however, said this idea had been tested and ruled out.

"This is why it is good to have two experiments [at the Tevatron]," said Soldner-Rembold, from the University of Manchester, UK. "What we see here is the scientific process at work. If one experiment sees something, another one has to verify it, and currently, we cannot verify it."

The Tevatron will be forced to shut down in September because of budget cuts in the US Department of Energy. By the time the machine accelerates its last particles, it will have been in operation for nearly 28 years. The decision had been taken despite the recommendations from a scientific panel that the Tevatron continue to operate for another three years in order to continue hunting for the "God particle."

The Tevatron is the only major competition to Europe's Large Hadron Collider machine. It is operated by the Fermi National Accelerator Laboratory (Fermilab), based in Batavia, Illinois.